Salt exclusion and up regulation of antioxidant scavenging system as mechanisms of salinity tolerance in rice

Three rice genotypes contrasting in salt-tolerance- IR29, IR65192-4B-10-3 (IR651), and FL478 were used to investigate the effects of salinity on plant growth, salt accumulation in plant tissue, and activities of enzymes and metabolites associated with reactive oxygen scavenging under salt stress [glutathione reductase (GR) and peroxidase (APX) activities, reduced ascorbate (AsA), malondialdehyde (MDA) and gene expression of OsAPx2 and OsAPx8]. Subjecting the three lines to salt stress for 18 days confirmed that IR29 was highly sensitive, whereas FL478 and IR651 were tolerant. Salt stress decreased green leaf area, and shoot and root lengths and their weights, with IR29 showing greater reductions. FL478 had the highest biomass production under NaCl stress which possibly helped in diluting the salts in the plant tissue. IR29 had the highest concentration of Na+ in the shoot (171% of the control), whereas IR651 showed the highest increase in the root (198% of the control). Maintenance of lower Na+ concentration in the shoots and storage of higher Na+ in the roots of tolerant genotypes suggest that salt exclusion at the root level or within the plant could play an important role in tolerance. High K+/Na+ ratio in the shoot is associated with salt tolerance. IR651 showed the highest concentration of reduced ascorbate (159ug/g FW), lowest MDA (12.9% of the control), in the shoot, and the highest increase in glutathione reductase (187% of the control) and APX (52%) activities. Conversely GR and APX activities and AsA were lower in IR29, which were reflected in greater cellular membrane damage as shown by the increase MDA content (31%) under NaCl stress. OsAPx8 expression was higher than OsAPx2 under saline conditions suggesting OsAPX8's possible role in salt tolerance. Tolerant genotypes seem to have more efficient anti-oxidative system and effective salt exclusion mechanisms than the sensitive genotype.